1. Field of the Invention
This invention relates to a method and apparatus for the drilling of a planar curved borehole. More particularly, this invention constitutes an improvement over the invention disclosed and claimed in the aforementioned prior patent.
2. The Prior Art
It is generally known that the final portion of a rotary drilling string (the so-called drilling collar) or the like is under compressive loads and torque during drilling, while the upper portion of the drilling string is under tension. As such, the weight of the drill collar below the point of last contact with the borehole wall may be thought of as being divided into two components; one acting along the axis of the collars and the second acting normal to the first, perpendicular to the borehole.
It is also generally known that, in principle, if the downhole orientation and magnitude of the normal force could be controlled during drilling, the drill bit could essentially be steered to any desired subsurface location or strata. Although theoretically possible and highly desirable, such a process and corresponding drilling equipment to achieve such a goal have not yet been developed. However, various processes and associated equipment have been employed that generally achieve varying degrees of what is recognized in the art as directional drilling. For example, it is common practice in oil and gas well drilling to use a so-called "whipstock" (a sloped plug inserted below the drill bit) to intentionally deflect the drill bit in a desired direction, thus creating a deviation in the direction of drilling.
Various types of drilling collar stabilizers have also been proposed to alter the direction of drilling. For example, U.S. Pat. Nos. 4,305,474 and 4,465,147 disclose stabilizers that create a deflecting force perpendicular to the drill string in order to control and guide the drill bit along a desired course of direction. Also, the use of an eccentric stabilizer has been proposed in U.S. Pat. No. 4,076,084 to drill a directionally oriented hole such as commonly practiced when drilling from an offshore platform or the like.
Another method and apparatus for drilling a curved borehole is disclosed in my prior U.S. Pat. No. 5,467,834, issued on a "METHOD AND APPARATUS FOR SHORT RADIUS DRILLING OF CURVED BOREHOLES". The present invention, in part, is an improvement over the invention disclosed in the aforementioned prior patent; the latter patent discloses a drilling sub located near the downhole end of a drilling string above the drill bit. The drill sub has an eccentric sleeve mounted thereon. The sleeve is provided with a retractable sidewall engaging means. The sidewall engaging means forces the drill string passing therethrough to one side of the well bore, thus lever arming the drill bit to the other side of the well bore by pivoting the lower end of the drill string. The retractable sidewall engaging means prevents the eccentric sleeve from rotating (in conjunction with the grooves on the opposite thin side of the sleeve) in the well bore, thus resulting in a curved well bore.
It is an object of the present invention to provide a downhole drilling method and apparatus which includes a drill string, a specialized drill sub mounted in the drill string adjacent the lower end thereof, an eccentric sleeve mounted on the drill sub, the sleeve being provided with a retractable sidewall engaging means and where and alignment device is mounted between the eccentric sleeve and the drill sub for permitting rotation of the drill sub relative to the eccentric sleeve after the retractable sidewall engaging means has been actuated to engage the side of the wellbore.
It is a further object of the present invention to provide a downhole drilling method and apparatus of the type referred to in the preceding paragraph wherein the permitted rotation of the drill sub serves to eliminate the effect of reactive torque from the drilling operation upon the eccentric sleeve.